There is a considerable market demand for waterproof leather for shoes, clothing or bags. However, the leathers are per se hydrophilic and most tanning agents and retanning agents as well as fatliquoring agents used in treating them are used by means of dispersants.
Most leathers used to date are chrome-tanned, owing to the particularly outstanding properties of this process. Only a few special leathers, such as sole leather, belt leather or leather for automotive dashboards, have been tanned with vegetable and/or with syntan tanning agents instead of with chromium.
However, the disadvantage of chrome-tanned leathers is considerable. Thus, for example, leather wastes which occur during the production of leather articles and leather at the end of the useful life present problems with regard to waste disposal, especially since chromium is no longer permitted to be stored on certain landfills. Moreover, chrome-tanned leather may develop traces of Cr(VI) under certain storage conditions or during incineration.
Other tanning systems which are on the market, such as, for example, aluminium, glutaraldehyde or tetrahydroxymethylphosphonium salt, have, apart from ecological problems, the disadvantage of compensating for chrome tanning salts with larger amounts of other generally hydrophilic tanning agents, such as vegetable extracts and synthetic, aromatic syntans, with resulting adverse effects on the waterproofness of the leather. Such substances undesirably increase the hydrophilicity of the leather, in a manner that makes the leather harder. In order to counteract this tendency, larger amounts of fatliquoring agents have to be used, which in turn have to be applied by means of emulsifiers, which may adversely affect waterproofness.
In the customary production process for waterproof leathers, chrome-tanned leather is treated with a combination of fatliquoring agents and water repellency enhancers, which are typically silicone emulsions. If this treatment is effected in water, use of emulsifiers for the water repellency enhancers is necessary, but adversely affects the waterproofness. In order to avoid the disadvantages of the hydrophilicity induced by emulsifiers in conventional, chrome-tanned systems, emulsification is carried out by means of compounds whose emulsifying, functional groups can be destroyed in a subsequent fixing step, usually with chromium salts.
The customary waterproofing compositions are non-sulphonated fatliquoring agents and water repellency enhancers which are fixed by means of mineral salts—as a rule with chromium(III). Such systems lead to open water repellency on chrome-tanned leathers, i.e., the generation of a hydrophobic network around the fibres without filling the spaces thereof.
However, when the customary waterproofing compositions are used on chromium-free leather, the desired waterproofness cannot be achieved because the amount of the chrome tanning agent has to be replaced by large amounts of hydrophilic vegetable tanning agents and/or syntans. The hydrophilic ingredients in the leather promote the penetration of water into the leather and not the opposite effect for water repellency.
The prior art is illustrated, for example, by H. Birkhofer, Reactive Hydrophobiermittel [Reactive Water Repellents], Das Leder [Leather] 1992, 71–75; Danisch, P. et al. Modern Hydrophobic Systems, JALCA, 1996, 120–125 Reiners, J. et al. Waterproofing of Leather, ALCA-Congress, Skytop, Pa., 2002 (to be published JALCA 2002).
It is therefore an object of the invention to provide waterproof leathers tanned without chromium.